1) Field of the Invention
The present invention relates to a transmitting apparatus on a communication network, preferably, to a transmitting apparatus on a radio communication network, and more particularly to a distortion compensation amplifying apparatus suitable for use in a transmitting unit constituting a base station, and it relates to a base station suitable for use in a radio network.
2) Description of the Related Art
FIG. 9 is a block diagram showing a transmitting apparatus in a base station which employs CDMA communications including a conventional digital pre-distortion compensation amplifying apparatus (digital pre-distortion AMP). In FIG. 9, a digital pre-distortion compensating amplifier 115 is connected to an antenna 116 to serve as a transmitting apparatus 117 to carry out radio transmissions of signals through the antenna 116.
This distortion compensation amplifier 115, shown in FIG. 9, is for use in a transmitting unit of a base station on a radio communication network, which interchanges signals with terminals according to the code division multiplex technique, and signals to the respective terminals are spectrum-spread in a spreading unit (not shown) and added so that a base band signal (which will be referred to hereinafter as a “BB signal”) undergoing the code division multiplex processing is inputted thereto.
In this configuration, although distortion occurs in a power amplifier 106, a signal is previously distorted to a non-linear distortion in the power amplifier 106 through the use of units 102 to 105 and 107 to 113 so that the distortion is canceled to accomplish the distortion compensation.
In this case, although it is desirable to attain a linear output characteristic with respect to an input the non-linear distortion causes a non-linear output characteristic. Due to the non-linear distortion, in particular the input becomes below a desired gain in a high-level output region, or the phase of an output signal rotates although it is desirable to provide an output signal in a state where the phase does not rotate with respect to an input signal. The distortion characteristic signifies this non-linear distortion characteristic.
A detailed description will be given hereinbelow of the configuration of the distortion compensating amplifier 115 shown in FIG. 9. The distortion compensating amplifier 115 is made up of a complex multiplier 102, a modulator (MOD) 103, a digital/analog converter (D/A) 104, a frequency converter (CONV) 105, a power amplifier (PA) 106, a directional coupler 107, an analog/digital converter (A/D) 108, a demodulator (DEM) 109, a delay unit (DL) 110, a subtracter 111, an address producing unit 112 and a lookup table (LUT) 113.
In addition, in the distortion compensating amplifier 115 shown in FIG. 9, a BB signal from a spreading unit is inputted to the main-signal-system complex multiplier 102 and further to a reference-signal-system address producing unit 112 and the delay unit 110. In the LUT 113, distortion compensation coefficients are accumulated to compensate for the distortion characteristic of the power amplifier 106 in accordance with the power of the inputted BB signal, and in the address producing unit 112, an address corresponding to the value of power of the BB signal is obtained so that a distortion compensation coefficient corresponding to the obtained address is outputted to the complex multiplier 102.
That is, the BB signal inputted to the complex multiplier 102 is outputted in a state complex-multiplied by the distortion compensation coefficient from the LUT 113. Following this, it undergoes the phase modulation processing in the modulator 103, the conversion processing from the digital signal into an analog signal in the digital/analog converter 104 and the conversion processing into a radio frequency signal in the frequency converter 105. After these processing, the signal is amplified in the power amplifier 106 and then transmitted as a transmitted signal through the antenna 116. The non-linear distortion characteristic in the power amplifier 106 is compensated through the use of the distortion compensation coefficient from the LUT 113 which is complex-multiplied with respect to the BB signal.
Moreover, it is known that the non-linear distortion characteristic in the power amplifier 106 varies according to the environment such as the temperature around the location thereof, and in the distortion compensating amplifier 115 shown in FIG. 9, for carrying out the distortion compensation corresponding to this environment variation, a signal outputted from the power amplifier 106 is feedbacked to update the distortion compensation coefficient stored in the LUT 113.
Concretely, a portion of the signal amplified in the power amplifier 106 is inputted through the directional coupler 107 to the frequency converter 115 to be converted from a radio frequency into a signal in the original baseband frequency band, and it is converted into a digital signal in the analog/digital converter 108 and demodulated in the demodulator 109. Moreover, in the subtracter 111, the demodulated signal undergoes the subtraction processing using a corresponding signal from the delay unit 110. Through the use of the output of this subtracter 111, there are detected only the non-linear distortion components of the actual input signal and output signal in the power amplifier 106. That is, in the LUT 113, the distortion compensation coefficient is renewable through the use of the distortion component from the subtracter 111. In other words, the renewal of the distortion compensation coefficient is made to suppress the distortion component to be detected.
As conventional techniques related to the present invention, there are the techniques disclosed in Japanese Patent Laid-Open Nos. HEI 9-69733 and HEI 9-102759.
Japanese Patent Laid-Open No. HEI 9-69733 discloses a distortion compensating amplifier including a means whereby a distortion compensation coefficient for the compensation for a distortion characteristic component is produced on the basis of an error signal between an input signal and an output signal to/from the amplifier according to an adaptable algorithm.
Japanese Patent Laid-Open No. HEI 9-102759 discloses a technique which employs a non-linear compensating method whereby a signal for the compensation of a non-linear characteristic of an amplifier is read out from a random access memory (RAM) and amplified after added to a modulated input signal in advance and which carries out the control so that, in a transmitter having a non-linear compensating circuit made to demodulate a portion of an amplifier output so that the RAM contents are renewed to become equal to the modulated signal, an output of a training signal generating circuit is connected to an input of the transmitter during an inactive period and the amplifier output is connected to a terminating circuit to make the renewal of the RAM contents through the use of the training signal.
However, in the case of the distortion compensating amplifier 115 shown in FIG. 9 and the technique disclosed in Japanese Patent Laid-Open No. HEI 9-69733, the distortion compensation is made through the control for reducing the difference between a transmitted signal and a feedback signal to the utmost, and with respect to the distortion compensation coefficient needed for the distortion compensation, the initial value is stored through the training according to a pattern (rating) determined in a factory under limited time, which makes it difficult to finely store the distortion compensation coefficient according to the input level.
Accordingly, for example, as indicated by a time point A in FIG. 10, in a case in which a signal remote temporarily from the rating during the operation and insufficient (high) in training is suddenly inputted to the power amplifier, difficulty is encountered in outputting an appropriate distortion compensation coefficient immediately and, because of the renewal of the table based on the distortion, it takes time until the distortion compensation coefficient reaches the convergence, so the degradation of the distortion characteristic occurs in the meantime.
In particular, for example, when many calls (connections between base station and terminal) occurs transiently during the operation on a radio network, the power increases rapidly and this further enhances the necessity to shorten the convergence time of the table for keeping the communication quality.
FIG. 11 is an illustration for explaining the addition of the signal power of each channel by the code division multiplex. In the case of the code division multiplexing, a plurality of channel signals are multiplexed in the same frequency band and, hence, as shown in FIG. 11, the respective channel signal powers are added to each other. Therefore, since the number of occupied channels increases as the call quantity increases, the signal power also rises. Moreover, irrespective of the degree of call quantity, if the information quantity communicated increases, the signal power rises accordingly.
In addition, taking into consideration the influence of a variation of the distortion characteristic stemming from aged deterioration of the power amplifier and temperature variation, although, when the call quantity is large at all times, the error quantity becomes small because a table has been produced to store distortion compensation coefficients in high-output conditions, in the normal actual operation in which the frequency of occurrence of a high output is relatively low, the error of the distortion compensation coefficient stored in the table become relatively large.
On the other hand, although it can be considered that tables are finely produced according to the magnitude (small, intermediate, large) of signal level at the training to shorten the table convergence, the test time in the factory becomes longer, which leads to an increase in cost.
Moreover, in the case of the technique disclosed in Japanese Patent Laid-Open No. HEI 9-102759, since a training signal can be transmitted only during the transmission downtime, for a communication network such as the code division multiplex access in which there is basically no need for the transmission downtime and the communications are made in a state where difficulty is encountered in securing such a time period, if the transmission power increases rapidly, difficulty is experienced in quickly achieving the table convergence, which makes it difficult to maintain the communication quality.